∞ generated and posted on 2016.02.28 ∞

Disease-causing microorganism, and particularly a disease-causing bacterium or virus.

Pathogens are microorganisms – particularly pathogenic bacteria, pathogenic fungi, or viruses – that, acting as parasites (rather than as, e.g., predators), harm other organisms such as ourselves.

A microorganism that causes harm to the hosts they infect, resulting in a reduction in the infected individual's health. Eukaryotic organisms that are capable of similarly impacting on the health of larger organisms are described instead as parasites and their "infections" as infestations.

The vast majority of bacteria, and even some viruses, are relatively harmless and therefore serve as commensals, that is, not harmful symbiotic organisms, rather than as pathogens. Indeed, a substantial fraction of bacteria are free living rather than typically found in symbiotic associations with larger organisms, such as ourselves.